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155. Min Stack

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Description

Design a stack that supports push, pop, top, and retrieving the minimum element in constant time.

Implement the MinStack class:

• MinStack() initializes the stack object.
• void push(val) pushes the element val onto the stack.
• void pop() removes the element on the top of the stack.
• int top() gets the top element of the stack.
• int getMin() retrieves the minimum element in the stack.

 

Example 1:

Input
["MinStack","push","push","push","getMin","pop","top","getMin"]
[[],[-2],[0],[-3],[],[],[],[]]

Output
[null,null,null,null,-3,null,0,-2]

Explanation
MinStack minStack = new MinStack();
minStack.push(-2);
minStack.push(0);
minStack.push(-3);
minStack.getMin(); // return -3
minStack.pop();
minStack.top();    // return 0
minStack.getMin(); // return -2

 

Constraints:

• -231 <= val <= 231 - 1
• Methods pop, top and getMin operations will always be called on non-empty stacks.
• At most 3 * 104 calls will be made to push, pop, top, and getMin.

Solutions

Python3

class MinStack:

    def __init__(self):
        """
        initialize your data structure here.
        """
        self.s = []
        self.helper = []


    def push(self, x: int) -> None:
        self.s.append(x)
        element = x if not self.helper or x < self.helper[-1] else self.helper[-1]
        self.helper.append(element)

    def pop(self) -> None:
        self.s.pop()
        self.helper.pop()

    def top(self) -> int:
        return self.s[-1]

    def getMin(self) -> int:
        return self.helper[-1]


# Your MinStack object will be instantiated and called as such:
# obj = MinStack()
# obj.push(x)
# obj.pop()
# param_3 = obj.top()
# param_4 = obj.getMin()

Java

class MinStack {

    private Deque<Integer> s;
    private Deque<Integer> helper;

    /** initialize your data structure here. */
    public MinStack() {
        s = new ArrayDeque<>();
        helper = new ArrayDeque<>();
    }

    public void push(int x) {
        s.push(x);
        int element = helper.isEmpty() || x < helper.peek() ? x : helper.peek();
        helper.push(element);
    }

    public void pop() {
        s.pop();
        helper.pop();
    }

    public int top() {
        return s.peek();
    }

    public int getMin() {
        return helper.peek();
    }
}

/**
 * Your MinStack object will be instantiated and called as such:
 * MinStack obj = new MinStack();
 * obj.push(x);
 * obj.pop();
 * int param_3 = obj.top();
 * int param_4 = obj.getMin();
 */

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